1. Field of the Invention
The present invention relates to a tractor-feed unit, mounted in a printer which prints text, images and the like, which feeds a continuous sheet.
2. Description of the Related Art
Conventionally, a continuous sheet used in a printer is provided with sprocket holes, which are arranged on both sides of the continuous sheet in order to maintain alignment in the longitudinal direction, perforated at a constant interval along the feed tracks. The printer is provided with a tractor-feed unit having protrusions engagable with the sprocket holes. The diameter of a protrusion is smaller than that of a sprocket hole so that the continuous sheet is easily fed onto the printer, and so that the continuous sheet may be easily removed from the printer after a printing operation.
The tractor-feed unit is disposed between a transfer unit and a fixing unit, to maintain a continuous sheet feed speed. The fixing unit has a pair of rollers which sandwich and feed the continuous sheet, allowing the feeding speed of the fixing unit to be varied in comparison with a feeding speed of the tractor-feed unit, which is essential because of the thickness of the sheet, roughness (i.e., friction coefficient) of the sheet, friction generated by a rubber roller of the fixing unit, and so on. Therefore, the rotational speed of the rollers is controlled so that tension generated in the continuous sheet located between the fixing unit and the tractor-feed unit is kept constant.
Since there is a gap between the sprocket holes of the continuous sheet and the protrusions of the tractor-feed unit, if the rotational speed of the rollers of the fixing unit is changed, to temporarily alter the feeding speed of the continuous sheet, the feeding speeds of the continuous sheet relative to the tractor-feed unit and the transfer unit are changed. As a result, a transfer offset occurs, and thus, the quality of the text or image printed on the sheet can be lowered. Further, if a disturbance or fluctuation occurs at the inlet of the printer, i.e., if the continuous sheet is pulled or disturbed in a direction opposite to the feed direction (see an arrow B shown in FIG. 1), the same problem described above would occur.